The Tavern

Is it possible for there not to be a Swamp entrance anywhere in the Lair? I have auto explored all eight levels but can't find it this game. Shoals and the Slime Pit are here, though.

All the lair branches aren't guaranteed, except for Slime Pit.

The Lair branches are randomly selected. Depending on the version you're playing, you'll get two of: Swamp, Shoals and Snake Pit or one of Swamp/Shoals and one of Snake Pit/Spider.

If you've only located Shoals and Slime, there's someplace you haven't explored yet. Have you tried every staircase and hatch? There could also be areas blocked off by plants.

each of the lair branches has a level range as well. If you press ctrl-O you will see where they might be (or where they are, if you found one but didn't notice).